The present invention relates to an arrangement for mitigating the effects of first- and second-order polarization mode dispersion (PMD) in optical fiber communication systems and, more particularly, to the recognition of the sources of second-order PMD and the provision of specific components that are capable of compensating for both first-order and second-order PMD.
Polarization mode dispersion (PMD) occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress or strain as well as random polarization coupling due to external forces acting upon the fiber. In particular, PMD causes optical signal distortion as a function of time. Consequently, PMD may severely impair the transmission of a signal in an optical fiber network. Indeed, with the continued push to higher bit rates (i.e., greater than 2.5 Gb/s) in telecommunication systems, PMD is becoming a non-negligible propagation effect. So-called xe2x80x9csinglexe2x80x9d mode fiber actually supports two modes, one for each polarization. Since in general the effective index of these two modes is not the same at any given point in a transmission system, there exists modal dispersion between the two polarizations (i.e., PMD).
It is well-known that PMD affects certain polarization components of an optical signal propagating through an optical fiber transmission line differently, such that differential time delays occur among the components as they travel through the fiber. These differential time delays may range from about 0.1 ps/(km)xc2xd for low-PMD optical fibers of modern manufacture to several ps/(km)xc2xd for single mode optical fibers of older manufacture. Disadvantageously, the differential time delay that may result over a xe2x80x9clong distancexe2x80x9d fiber optic link (for example, a 100 km terrestrial transmission system) may be more that 20 ps, with a 10 ps or greater delay associated with a transoceanic link employing state-of-the-art low-PMD optical fiber.
It is well-known that the differential time delay that might occur in a particular transmission fiber is not constant over time, but may vary as the physical environment of the fiber changes (e.g., temperature of the fiber, pressure upon the fiber). Thus, the statistics of time-dependent differential time delay caused by PMD in an optical fiber usually follows a Maxwellian distribution and, therefore, at any point in time, may be substantially lower to several times higher than its average (or mean) value.
Prior methods of dealing with signal impairments associated with PMD in an optical fiber include, for example: (1) electrical equalization of the signal distortion caused by PMD, as discussed in an article entitled xe2x80x9cExperimental Equalization of Polarization Dispersionxe2x80x9d, by M. A. Santoro et al., appearing in IEEE Photonic Technology Letters, Vol. 2, No. 8, 1990, beginning at page 591; and (2) electrical compensation of the differential time delay in the received electrical signals, as discussed in the article entitled xe2x80x9cPolarization Mode Dispersion Compensation by Phase Diversity Detectionxe2x80x9d, by B. W. Hakki, appearing Photonic Technology Letters, Vol. 9, No. 1, 1997, beginning at page 121. Such prior methods also include optical compensation of the differential time delay before converting the optical signals into electrical signals, as discussed in the article xe2x80x9cAutomatic Compensation of First-Order Polarization Mode Dispersion in a 10-Gb/s Transmission Systemxe2x80x9d, by F. Heismann et al, appearing in the Proceedings of ECOC ""98, September 1998.
While these methods are useful at addressing first-order effects, there remains in the art the need to address the impact of second-order polarization mode dispersion on optical fiber transmission systems.
The need remaining in the prior art is addressed by the present invention, which relates to an arrangement for mitigating the effects of first- and second-order-polarization mode dispersion (PMD) in optical fiber communication systems and, more particularly, to the recognition of the sources of second-order PMD and providing components that are capable of compensating for first-order and second-order PMD.
In accordance with the teachings of the present invention, first-order PMD compensation can be provided by any technique well-known in the art, such as by including a time delay arrangement that is controlled to adjust the propagation differential between the orthogonal polarization states. Second-order PMD is recognized in accordance with the present invention as associated with at least one of three distinct effects: (1) polarization dependent pulse broadening (analogous to chromatic dispersion); (2) additional pulse broadening (attributed to optical filtering); and (3) coupling of a portion of the optical signal into the orthogonal polarization (relative to the main pulse) with a different propagation time. Each one of these effects is increased when the spectral bandwidth of the optical signal is increased. In one embodiment of the present invention, therefore, second-order PMD can be minimized by minimizing the spectral bandwidth of the transmitted pulse.
In an alternative embodiment of the present invention, second-order PMD is compensated by including separate elements within the transmission system that address each of the three identified sources of second-order PMD mentioned above. For example, a chirped fiber grating with a variable temperature gradient can be used to compensate for the first type of pulse broadening that is akin to chromatic dispersion. An independent, complementary optical filter with variable spectral transmission can be inserted in the signal path to compensate for the additional pulse broadening. Lastly, an additional polarizer can be inserted in the transmission path to filter out the signal that has coupled into the unwanted polarization.
In accordance with the present invention, the various components used to provide the above-described compensation may be disposed in any suitable arrangement along the signal path. Additionally, one may sacrifice the degree of second-order compensation (if size or economy is an issue, for example) by eliminating one or two of the three separate components used for second-order PMD compensation.